The present invention relates to injection molding apparatus in general, and more particularly to improvements in apparatus for controlling the pressure of fluid medium which causes or allows the plasticizing screw to move lengthwise in the barrel of an injection molding machine.
It is known to employ in an injection molding machine a hydraulic cylinder wherein a piston moves back and forth under or against the action of oil or another hydraulic fluid to effect or allow axial movements of a plasticizing screw which is surrounded by a barrel having one or more nozzles for injection of plasticized synthetic plastic material into the cavity or cavities of a mold. The cylinder can receive pressurized hydraulic fluid from one or more motorpump aggregates, and the pressure of fluid in the cylinder is regulated by valves so as to maintain the fluid under a relatively low basic pressure when the screw moves away from the nozzle, to maintain the fluid under a higher injection pressure when the screw begins and continues to move toward the nozzle, and to maintain the fluid at a highest holding pressure not later than when the screw approaches and reaches its front end position at a minimum distance from the nozzle. The screw is rotatable by a suitable motor or transmission so that it can subject freshly admitted granulate to a plasticizing action and simultaneously forces plasticized material into the space between its front end and the nozzle of the barrel in the injection molding apparatus. The barrel receives fresh granulate through the medium of a feed hopper. The length of the rotatable and reciprocable screw is selected in such a way that its front end face is located at a predetermined minimum distance from the nozzle upon completion of each injection molding cycle, i.e., when the screw has completed its axial movement toward the nozzle and has expelled a predetermined quantity of plasticized material through the nozzle and into the cavity or cavities of the mold. The space between the nozzle and the front end face of the screw is then filled with a cushion of plasticized material. The next injection molding cycle is started by admitting fresh granulate into the hopper whereby the granulate enters the barrel and is plasticized by the rotating screw which causes the plasticized material to augment the aforementioned cushion. The material of the cushion cannot escape through the nozzle so that the screw is pushed back and causes the piston which is connected to the ram or piston rod for the screw to move backwards and to expel hydraulic fluid from one chamber of the cylinder. The fluid which fills such chamber is then maintained at the relatively low basic pressure which cannot prevent the screw from moving axially in a direction away from the nozzle at the front end of the barrel. The pressure of hydraulic fluid in the aforementioned chamber of the cylinder is increased to injection pressure when the screw reaches a predetermined rear end position; the screw is then moved forwardly by the ram and piston so that it expels plasticized material from the barrel, and such material penetrates into the cavity or cavities of the mold. Shortly before the screw reaches its front end position, the pressure of hydraulic fluid is raised or lowered to holding pressure, and such pressure is maintained during the last stage of the injection molding cycle.
The making of satisfactory injection molded articles necessitates an extremely accurate control of fluid pressures in the cylinder which controls axial movements of the plasticizing and material injecting screw. Such control includes the magnitude of each of the aforediscussed basic, injection and holding pressures as well as the length of each interval during which the fluid is maintained at a particular pressure. Only such mode of operation insures that the cavity or cavities of the mold are properly filled with material which, when hardened, forms an injection molded article of desirable color, size, shape and strength. The deviations of the weight and/or dimensions of injection molded articles from an optimum weight and/or dimensions are attributable chiefly, or to a large extent, to improper selection of injection pressure and/or to improper selection of the speed at which the screw is caused to move forwardly while the fluid in the cylinder is maintained at injection pressure.